Car Body Paint Touch-Up / Surface Prep

I've got a '95 Jeep Grand Cherokee Laredo that we've taken good care of, and/or just been remarkably lucky with owning - as it's had very few problems mechanically, and its black paint still has a near new appearance.

I've had a mostly great experience recently touching-up, or simply outright painting a couple of replacement parts (for this vehicle and others), that were pre-primed and off the vehicle. I figure that's about 9 parts attributable to the pre-primed parts and just general improvements to the OEM paint formulation and delivery nozzles, and about 1 part having to do with my expertise.

I'm getting ready to paint the only place on my Jeep, where for some reason the paint job has clearly begun to fail (the left, front upper surface of the quarter panel), but virtually nowhere else.

These pictures would seem to indicate that the clear coat and the paint have begun to oxidize and fail, but the primer seems to be holding up okay.




Since this oxidation and breakdown of the paint is just on the upper surface, I thought I would mask off and repaint that area, and just down vertically on the panel, where there's a convenient horizontal crease in the panel, that will help "blend" where the repainting ends, and the touch-up paint begins. Since the last time I can recall working on this kind of on-car touch-up, was helping my dad repaint the rocker panels on our '62 Rambler station wagon (when I was 8 years old, no less . . .), I thought I would ask for some help on the surface prep that's needed, in order to make this touch-up appear reasonably decent when it's finished.

What sort of "sanding" (or whatever) is needed to give the surface area a more flat, even surface, to accept paint - maybe without compromising the primer coat which seems to be in perfect, intact condition. Is that wet-dry sandpaper, steel wool, or will just abrasive language directed at the panel do the trick? (I suspect that last option will not do the trick, but it seemed like fun to say it . . .)